The in vivo stability of tolmetin-plasma protein adducts was characterized in six healthy human volunteers after a 400 mg single dose and after a multiple-dose regimen of 400 mg tolmetin every 12 hours for 10 days. Although the mean +/- SD maximum bound concentration was only 2.72 +/- 0.98 ng drug/mg protein after a single dose, it was almost an order of magnitude higher after multiple dosing. The protein adduct exhibited an average half-life of 4.8 +/- 0.9 days in contrast to the much shorter 5-hour half-lives for tolmetin and its glucuronide.